Electronic filters, such as simple RC filters, for example, are sometimes used in applications where it is desirable to change the characteristics of the filter. For example, a particular filter may work best with a fast time constant under certain circumstances, but under different circumstances a relatively slow time constant may be required. Switching one or more impedances into or out of circuit with the filter is one way to effectively change the filter's characteristics. However, in applications where the filter and its associated circuitry are constructed using semiconductors of the bipolar type (as opposed to MOS types), it is very difficult to construct a bipolar-type switch that does not introduce an undesirable high level of voltage offset.
The foregoing switching problem and its attendant offset are issues that confront the designers of bipolar automotive voltage regulators, because such regulators use filters that could benefit from a truly switchable filter with no discernable offset. Accordingly, the following discussion will describe this issue as it pertains to automotive voltage regulators.
The typical electronic regulator modifies the excitation applied to an alternator's field winding so as to hold the alternator's output voltage close to a desired mean value. This is usually achieved by periodically pulse-width-modulating the excitation voltage applied to the field winding so that the "on" time of the excitation voltage varies over a given regulation cycle (each regulation cycle may typically last for about 20 milliseconds). Where a larger alternator output voltage is called for, the "on" time of the excitation voltage is increased.
As described in more detail below, switching the excitation voltage off and on causes relatively large and undesired voltage steps to appear in the output of the alternator that is sensed by the regulator. These steps, plus other undesired transient voltages, are conventionally applied to a filter (typically, an R-C filter) prior to being processed by the regulator. If the filter is not able to quickly recover from the switching-induced voltage steps, improper regulation can occur. One undesirable result of such improper regulation is an apparent reduction of the regulation frequency from 50 Hz (typically) to 25 Hz. This change in frequency can manifest itself in visible headlight flicker, an obviously objectionable result.
As mentioned above, the cause of this problem is the inability of the regulator's filter to quickly recover from the switching-induced voltage steps. This recovery should preferably occur during the regulator's minimum "on" time (a minimum interval when excitation is always being applied to the alternator's field winding) which may be as small as 1 millisecond. Thus, a filter with a fast time constant is called for. On the other hand, the filter's time constant needs to be relatively long to effect good regulation under all operating conditions. Both of these concerns could be met by a switchable filter, but no such filter, in bipolar form, has been available except for those which introduce too much offset to be practical.